Diketopiperazines have been reported to exhibit a variety of biological activities. See, e.g., U.S. Pat. No. 3,941,790 (cancer treatment), U.S. Pat. No. 4,289,759 (immunoregulatory agents), U.S. Pat. No. 4,331,595 (immunoregulatory agents), U.S. Pat. No. 4,940,709 (platelet activating factor (PAF) antagonists), U.S. Pat. No. 5,700,804 (inhibitors of plasminogen activator inhibitor), U.S. Pat. No. 5,750,530 (inhibitors of plasminogen activator inhibitor), U.S. Pat. No. 5,990,112 (inhibitors of metalloproteases), U.S. Pat. No. 6,537,964 (chemosensitizing reversal agents for treatment of multiple drug resistant cancers), U.S. Pat. No. 6,555,543 (inhibitors of PAF, the production and/or release of interleukin 8 (IL-8) and inflammation), and U.S. Pat. No. 6,815,214 (treatment of inflammatory conditions associated with, e.g., cancer and asthma), PCT applications WO 97/36888 (inhibitors of farnesyl-protein transferase), WO 98/9968 (treatment for infections, cancer and other malignant diseases), WO 99/40931 (treatment of central nervous system injury), and WO 04/87162 (agents for treatment of drug resistant cancer), EP application 43219 (immunoregulatory agents), Japanese application 63 290868 (PAF antagonists), Japanese application 31 76478 (immunosuppressive agents), Japanese application 51 63148 (anti-neoplastic agents), Shimazaki et al., Chem. Pharm. Bull., 35(8), 3527-3530 (1987) (PAF antagonists), Shimazaki et al., J. Med. Chem., 30, 1709-1711 (1987) (PAF antagonists), Shimazaki et al., Lipids, 26(12), 1175-1178 (1991) (PAF antagonists), Yoshida et al., Prog. Biochem. Pharmacol., 22, 68-80 (1988) (PAF antagonists), Alvarez et al., J. Antibiotics, 47(11), 1195-1201 (1994) (inhibitors of calpain).